In situ SHRIMP U-Pb dating of monazite integrated with petrology and textures: Does bulk composition control whether monazite forms in low-Ca pelitic rocks during amphibolite facies metamorphism?

Bulk composition and specific reaction history among common silicate minerals have been proposed as controls on monazite growth in metapelitic rocks during amphibolite facies metamorphism. It has also been implied that monazite that formed during greenschist facies metamorphism may be preserved unchanged under upper amphibolite facies conditions. If correct, this would make the interpretation of monazite ages in polymetamorphic rocks exceedingly difficult, because isotopic dates could vary significantly in rocks that have experienced identical metamorphic conditions but differ only slightly in whole-rock composition. Low-Ca pelitic schists from the Mount Barren Group in southwestern Australia display a range of whole-rock compositions in AFM space and different peak mineral assemblages resulting from amphibolite facies metamorphism (∼8 kb, 650 °C). In this study, we test whether bulk composition controls the formation of monazite through geochronology and textural evidence linking monazite growth with deformation and peak metamorphism. X-ray element mapping of monazite from the metapelitic rocks reveals concentric zoning in many grains with compositionally distinct cores and rims. In situ SHRIMP U-Pb geochronology of monazite yields two ²⁰⁷Pb/²⁰⁶Pb age populations. The cores, and texturally early monazite, give an age of 1209 ± 10 Ma, interpreted to record prograde metamorphism, whereas the rims and “late” monazite define a single population of 1186 ± 6 Ma, which is considered the likely age of peak thermal metamorphism. The growth of monazite was widespread in low-Ca pelitic schists representing a broad range of compositions in AFM space, indicating that variations in bulk composition in AFM space did not control the formation of monazite during amphibolite facies metamorphism in the Mount Barren Group.     

Aeolian mass transport near the saltation threshold

Aeolian mass transport was investigated in a desert field experiment where the wind speeds were near the saltation threshold. Bed transport was observed during 45 min runs even though the calculated values of bed shear stress using conventional laboratory equations for mass transport predicted that there should be no transport. We therefore investigated the possibility of predicting mass transport using quasi-instantaneous wind speeds, i.e. values derived at a time scale similar to that of the saltation process. Quasi-instantaneous wind speeds are able to predict mass transport associated with the stronger gusts. Predicted mass transport values compare fairly well with observation, but the accuracy of the prediction is very sensitive to correct estimation of the surface roughness and the saltation threshold for the particular sand bed. When these values differ by only 10 percent from the values that optimize the estimation, predicted mass transport can differ by up to ±50 per cent. Copyright © 1999 John Wiley & Sons, Ltd.     

Detailed mapping of marine erosional surfaces and the geometry of clinoforms on seismic data: a tool to identify the thickest reservoir sand

Deltaic sediments of the Billund and Bastrup sands were deposited in a ramp setting in the storm-dominated North Sea during the early Miocene. A marked relief in the hinterland and the relatively high precipitation resulted in a high sediment supply to the sea and progradation of major delta-coastal plains south of the present-day Norway. The focus of this study is on the forced regressive wedge system tracts of the two delta complexes, which show remarkably well-developed marine erosional surfaces associated with sand-rich packages characterised by steeply dipping clinoforms (up to 10°). The well-developed clinoformal packages indicate that deposition occurred in water depths of 60–100 m even under a sea-level fall. The sand-rich delta lobes also demonstrate that it was a high-energy environment and that wave-generated re-suspension at the delta front effectively re-sorted the sediments and sand-rich systems became separated from mud-dominated portions of the delta complexes. The evolution of the above occurred in a basin that has been exposed by inversion tectonism. The sediment supply was consequently high. During deposition, eustatic sea-level changes strongly controlled the evolution of sequences. The results found in this study may be applicable for mapping reservoir sands in ramp settings and in rift basins especially when looking for reservoir rocks in the basinal setting or when carrying out detailed reservoir mapping in already existing hydrocarbon fields.     

Postseismic Deformation Following the 1994 Northridge Earthquake Identified Using the Localized Hartley Transform Filter

Here we present a new mathematical tool, the localized Hartley (HL) transform (Hartley, 1942; Bracewell, 1990), that allows for the filtering of 1-D time series through the identification of the power at various spatial and temporal wavelengths. Its application to and the associated results are presented from its application to continuous Global Positioning System (GPS) data from southern California for the time period 1994 through 2006. The HL transform filter removes the high-frequency components of the data and effectively isolates the longer period signal. This long-period signal is modeled as time-dependent postseismic deformation using the viscoelastic-gravitational model of Fernández and Rundle (2004) for six stations selected for their proximity to the Northridge earthquake. The x-, y-, and z-components of the postseismic deformation are compared to the filtered data. Results suggest that this long-period deformation is a result of postseismic relaxation and that the HL transform filter provides an important new technique for the filtering of geophysical data consisting of the superposition of the effects of numerous complex sources at a variety of spatial and temporal scales.     

Derivation of streamflow statistics based on a filtered point process

This work presents the derivation of general streamflow cumulants from daily rainfall time series. The general streamflow cumulants can be used to compute basic streamflow statistics such as mean, variance, coefficient of skewness, and correlation coefficient. Streamflow is considered as a filtered point process where the input is a daily rainfall time series assumed to be a marked point process. The marks of the process are the daily rainfall amounts which are assumed independent and identically distributed. The number of rainfall occurrences is a counting process represented by either the binomial, the Poisson, or the negative binomial probability distribution depending on its ratio of mean to variance. The first three cumulants and the covariance function of J-day averaged streamflows are deduced based on the characteristic function of a filtered point process. These cumulants are functions of the stochastic properties of the daily rainfall process and the basin-response function representing the causal relationship between rainfall and runoff.     

The effect of wind speed and bed slope on sand transport

This paper reports on a wind tunnel study of the effects of bed slope and wind speed on aeolian mass transport. The use of a sloping wind tunnel has enabled estimation of the friction angle α to be about 40° for saltating particles in the range 170–540 μm. A formula relating dimensionless mass transport to friction speed and bed slope is proposed, and mass transport data for five uniform sand samples and one non-uniform sand sample are shown to fit the equation well. In particular, the relationship reveals an overshoot in mass transport slightly above threshold collisions, a feature also evident when previous experimental data is re-examined. As the number of mid-air collisions between the saltating particles increases greatly with wind speed, the overshoot may occur as a result of increasing energy losses resulting from the collisions. Finally, it is demonstrated that data for saltating snow shows a similar overshoot in the dimensionless transport rate.     

Supporting Mongolian pastoralists by using GIS to identify grazing limitations and opportunities from livestock census and remote sensing data

Since 1990, Mongolia has experienced a radical change away from centralized livestock production to more traditional rangeland management practices. As the herders now have increased access to the pastures, they need to be able to evaluate the sustainable level of exploitation of the rangeland. This paper demonstrates how pertinent information on the state of the rangeland resources can be made available to herdsmen by using a Geographical Information System (GIS). The focus is on the importance of having a sound data and information framework when assessing rangeland resources. The three main requirements are: first, knowledge of the production system; second, a natural resource inventory; and third, an assessment of the natural resource exploitation. Workshops held in the field brought together herdsmen, administrators, scientists and project personnel to identify and discuss issues of range management. From topographic maps, a digital elevation model was created using GIS, which together with a recent land-cover map elaborated from a SPOT satellite image made it possible to map the important areas suitable for winter grazing. The exact locations of the family winter settlements were recorded and linked to annual livestock statistics using GIS to identify the areas being grazed and to calculate the stocking rates by household. It was shown for the administrative unit of Arbayasgalan that the ratio of stocking rates to carrying capacity exceeded one, which indicates overstocking. However, the uneven distribution of grazing pressure over the study area enabled the proposal of actions to mitigate serious overgrazing. A discussion of range management practices was facilitated by providing the herdsmen with information on the extent and location of the problem. This revised version was published online in July 2006 with corrections to the Cover Date.     

Hydrothermal uranium deposits containing molybdenum and fluorite in the Marysvale volcanic field, west-central Utah

Uranium deposits containing molybdenum and fluorite occur in the Central Mining Area, near Marysvale, Utah, and formed in an epithermal vein system that is part of a volcanic/hypabyssal complex. They represent a known, but uncommon, type of deposit; relative to other commonly described volcanic-related uranium deposits, they are young, well-exposed and well-documented. Hydrothermal uranium-bearing quartz and fluorite veins are exposed over a 300 m vertical range in the mines. Molybdenum, as jordisite (amorphous MoS₂), together with fluorite and pyrite, increase with depth, and uranium decreases with depth. The veins cut 23-Ma quartz monzonite, 20-Ma granite, and 19-Ma rhyolite ash-flow tuff. The veins formed at 19-18 Ma in a 1 km² area, above a cupola of a composite, recurrent, magma chamber at least 24 × 5 km across that fed a sequence of 21- to 14-Ma hypabyssal granitic stocks, rhyolite lava flows, ash-flow tuffs, and volcanic domes. Formation of the Central Mining Area began when the intrusion of a rhyolite stock, and related molybdenite-bearing, uranium-rich, glassy rhyolite dikes, lifted the fractured roof above the stock. A breccia pipe formed and relieved magmatic pressures, and as blocks of the fractured roof began to settle back in place, flat-lying, concave-downward, “pull-apart” fractures were formed. Uranium-bearing, quartz and fluorite veins were deposited by a shallow hydrothermal system in the disarticulated carapace. The veins, which filled open spaces along the high-angle fault zones and flat-lying fractures, were deposited within 115 m of the ground surface above the concealed rhyolite stock. Hydrothermal fluids with temperatures near 200 °C, ¹⁸O_H2O∼−1.5, D_H2O∼−130, log f O₂ about −47 to −50, and pH about 6 to 7, permeated the fractured rocks; these fluids were rich in fluorine, molybdenum, potassium, and hydrogen sulfide, and contained uranium as fluoride complexes. The hydrothermal fluids reacted with the wallrock resulting in precipitation of uranium minerals. At the deepest exposed levels, wallrocks were altered to sericite; and uraninite, coffinite, jordisite, fluorite, molybdenite, quartz, and pyrite were deposited in the veins. The fluids were progressively oxidized and cooled at higher levels in the system by boiling and degassing; iron-bearing minerals in wall rocks were oxidized to hematite, and quartz, fluorite, minor siderite, and uraninite were deposited in the veins. Near the ground surface, the fluids were acidified by condensation of volatiles and oxidation of hydrogen sulfide in near-surface, steam-heated, ground waters; wall rocks were altered to kaolinite, and quartz, fluorite, and uraninite were deposited in veins. Secondary uranium minerals, hematite, and gypsum formed during supergene alteration later in the Cenozoic when the upper part of the mineralized system was exposed by erosion. Received: 23 June 1997 / Accepted: 15 October 1997     

Enhancement in wind-driven sand transport by electric fields

Aeolian grain transport is a powerful erosion mechanism and a significant factor affecting atmospheric dynamics by the creation of particulate aerosols. Electric fields, both natural and man-made, occur widely at the Earths surface in environments where granular material is found. Such electric fields may induce electrification of granular material and affect the transport dynamics of the grains. In this laboratory study we show that the influence of such electric fields is significant at field strengths well below that at which breakdown occurs and present a simple semi-empirical expression which allows this mechanism to be quantified.